Ongoing design improvements are leading to increasing reliability, robustness and most importantly, greater efficiency of electrical rotating machinery. Coupled with improvements in the methods of controlling these machines, these design changes have led to the increasing usage of electric motors and generators in rotating machinery. In parallel with the increasing relevance of electrical rotating machinery, has been the development of more considered approaches of monitoring the condition of motors and generators. Typically, the health of these machines has been evaluated on the basis of measured temperatures and vibrations.
Recently, the analysis of stator winding currents, typically measured from the power cables connecting the electrical rotating machines to the power source has been grown in recognition as a method for identifying the condition of electrical machines, as well as some certain other components which form part of the shaft line. Spectral analysis of these measured stator winding currents, more typically known as Motor Current Signature Analysis (MCSA), represents a widely adopted method of analyzing the health of electrical rotating machinery using signals measured from power cables. Faults in the electrical machine result in load variations as well as variations in the size of the air-gap between the rotor and the stator. Both of these variations serve to modulate the large alternating current (AC) power supply currents, resulting in sidebands of the supply frequency to be visible in the frequency spectrum of the current. Specific fault modes may be related to specific frequencies in the current spectrum; hence it is possible to identify the type and severity of a fault in an electrical machine.
There are various advantages of MCSA. In addition to being comparatively cheap to implement, there are inherent advantages associated with directly measuring signals from the power cables connecting the electrical rotating machines to the power source. Firstly, current and voltage sensors may be considered as non-invasive as the electric rotating machine forms part of the electromechanical system. Secondly, transmission path effects associated with the location of the transducer relative to a fault are less severe.
Whilst MCSA represents a powerful tool, it is not without its limitations. An often undisclosed assumption in this analysis is that the phase voltages supplied to the electric machine are idealized sinusoids that are not influenced by the condition of the machine. In reality, back electromagnetic forces (EMFs) generated in a machine are likely to differ between faulty and healthy machines, and hence the voltages measured at the motor terminals are likely to include some dynamic signatures which may be interrogated for diagnostic information. Whilst it is true that in the case of a machine supplied direct-on-line the dynamic signatures in current signals are much more easily discerned than the equivalent signatures in the voltage, it is also true that by neglecting voltage measurements, some information relevant for condition monitoring is also neglected. This situation is particularly relevant in the case of electrical machines which are supplied by a drive, where controller actions can act to transfer information from the current signals to the voltage supplied to the machine. Thus, methods that combine both currents and voltages measured from power cables connecting the electrical rotating machines to the power source can ensure that no potentially useful diagnostic information is ignored. An example of such a method is the analysis of electromagnetic torques estimated on the basis of measured currents and voltages. However, methods of estimating the torque of a machine require accurate estimates of machine parameter values such as the stator resistance. Such values are not always easily available and their accurate estimation is non-trivial.
Measured currents and voltages may be combined to obtain the admittance or impedance of the electric machine, which in turn may be analyzed to identify the health of the machine. US patent description US2007/0282548A, describes a method for determining conditions of mechanical system driven by a motor by monitoring an admittance or impedance at an input to the motor over a period of time. The admittance or impedance is calculated using current and voltage measurements. Variation in admittance or impedance is associated with known conditions including faults. Whereas US patent description US2007/0282548A describes a method of deriving and analyzing the admittance or impedance of a single supply phase, it is often the case that a greater amount of diagnostic information may be obtained by comparing the differences between the separate phases of a polyphase rotating electrical machine. An example of where this approach may prove to be a limitation is in the case of diagnosing static eccentricity in a three-phase electrical machine. In this situation, whilst certain characteristics of the impedance estimated from currents and voltages measured from two of the three phases will differ from the equivalent impedance estimates from a healthy machine, in the third phase the difference between the impedance estimated in the healthy case and in the case with static eccentricity may be negligible. As a result, there is a risk of monitoring approaches based upon impedance estimated from currents and voltages measured from only one phase of an electrical machine being insensitive to certain developing faults, with the potential for many missed alarms